The present invention relates to a screw fastener comprising a threaded shank having a force application means for torque transmission and a screw fastener point. The threaded shank consisting of a shank core and a self-tapping thread, and the thread being designed as a prominence which runs helically over the shank core, is defined by two flanks meeting in an outer thread edge and has a height measured radially between the core and the thread edge. The outer thread edge, at least in a section of the thread, running in a wavelike manner in the radial direction with a certain amplitude between wave crests having the thread height and wave troughs having a height reduced by the amplitude, and the thread, at least in the region of one of its flanks, having niches in the region of the wave troughs of the thread edge. The niches interrupt the surface of the flank and their outer boundary is the thread edge, the thread in each case having a certain first apex angle, formed between the flanks, in those regions of the wave crests of the thread edge which are not interrupted by niches and a second apex angle in the deepest regions of the wave troughs of the thread edge.
Such a screw fastener is described in DE 33 35 092 A1. It has proved to be very effective in practice, since a high release torque at a low insertion torque is achieved. The thread runs up to the end of the screw fastener point, it being designed with the niches and the wavelike thread edge, starting from the screw fastener point, at least over the first adjoining thread turn. As a result, the point acts as a type of reaming tool, the thread forming being effected directly at the point, so that reliable centering and biting in the workpiece results directly when applying the screw fastener. In this known screw fastener, the niches are formed as paraboloids symmetrically to the center line of the wavelike thread edge.
EP 0 394 719 B1 likewise describes a thread-forming screw fastener of the generic type, in which, however, the niches are designed to be asymmetrical in such a way that their front flank surfaces in the insertion direction are steeper than the rear flank surfaces in the insertion direction. As a result, a further reduction in the insertion torque with a simultaneous increase in the release torque is achieved. During the insertion, the resistance due to the flatter design of the rear parabolic parts in the insertion direction is lower, whereas the release of the screw fastener is made more difficult on account of the steeper arrangement of the parabolic areas lying at the front in the insertion direction.
The object of the present invention is to improve a screw fastener of the generic type in such a way that the insertion torque is reduced even further, but a high release torque is also ensured at the same time. In this case, the screw fastener is to be conceived with optimized properties, on the one hand for inserting into softer materials, such as wood and the like, in particular without pilot drilling and thus in a self-piercing manner, or on the other hand for inserting into harder materials, e.g. plastics and metals, in particular into a core hole.